1. Field of the Invention
The present invention is directed to a digital hearing aid having a variable directional microphone characteristic, and to a method for operating such a digital hearing aid, of the type having a signal processing unit and earphone, and at least two microphones.
2. Description of the Prior Art
Hearing aids are known wherein the signal transit times in the individual microphone signal paths are modified mechanically or by circuitry (for example, by low-pass filters) for producing a variable directional microphone characteristic. A signal delay in the DSP (digital signal processor) stage is known for digital hearing aids. For realizing a more finely graduated adjustment of the directional microphone characteristic in digital hearing aids, however, a substantially higher sampling frequency would have to be employed and this could only be realized with considerable structural outlay and power consumption. Due to the real-time demands in the signal path, moreover, undesired falsifications of the signal to be processed would occur.
German PS 195 45 760, corresponding to U.S. Pat. No. 5,796,848, discloses a digital hearing aid with a microphone, a signal processing unit and an earphone, wherein a sigma-delta modulator as well as a following interpolation filter are provided in the microphone signal path, and whereinxe2x80x94for protecting against electromagnetic emissionxe2x80x94an analog-to-digital converter is provided in the microphone housing and a digital (decade) filter and a signal processor are provided between the sigma-delta modulator and the interpolation filter.
An object of the present invention is to provide a digital hearing aid as well as a method for producing a variable directional microphone characteristic wherein a finely graduated adjustment of the directional microphone characteristic can be realized with little structural outlay.
The above object is achieved in accordance with the principles of the present invention in a digital hearing aid with a variable directional microphone characteristic having a signal processing unit, an earphone and at least two microphones each having a microphone signal path associated therewith, and wherein a sigma-delta converter immediately followed by a delay element are provided in at least one of the microphone signal paths.
The above object is also achieved in accordance with the principles of the present invention in a method for operating a digital hearing aid having a signal processing unit, an earphone and at least two microphones each having a microphone signal path associated therewith, including the step of undertaking an analog-to-digital conversion of the signal in at least one of the microphone paths using a sigma-delta converter, thereby resulting in a 1-bit output signal from the sigma-delta converter, and delaying the 1-bit output signal.
As a result of the sigma-delta converter as well as the delay element following immediately thereafter which are provided in at least one microphone signal path of the inventive hearing aid, a sampling of the microphone signal to be delayed can ensue with a sampling frequency which is significantly increased compared to known hearing aids and processing methods, allowing substantially more finely graduated delays to be achieved. An arbitrarily finely graduated transition thus can be achieved in the adjustment of the directional microphone characteristic, allowing arbitrary intermediate forms of the microphone to be realized, for example characteristics transitioning from a xe2x80x9cFigure eightxe2x80x9d characteristic to a xe2x80x9ckidney-shapedxe2x80x9d characteristic. In the inventive hearing aid, thus, a desired directional microphone characteristic can be set, individually suited to a current audio environment or a user preference.
A DSP control or a read-only memory can be allocated to the delay element.
Advantageously, an interpolation filter, preferably operating as low-pass filter, is provided in at least one microphone signal path in order to be able to attenuate signal disturbances as may occur when switching and shifting the directional microphone characteristic. As a result the transition between the individual steps of the directional microphone characteristic can be quasi-continuously designed.
In another embodiment, the inventive hearing aid has a first microphone signal path with a sigma-delta converter and an interpolation filter. Further, a second microphone signal path is provided that, following a sigma-delta converter, has a first signal path branch with a delay element and an interpolation filter and a second signal path branch arranged parallel to the first signal path branch and having a further interpolation filter.
By combining the signals of the first microphone signal path with the signals of the first or second signal path branches, different directional microphone characteristics can be achieved in respective summing elements, since a different directional microphone characteristic arises in each of the summing elements due to the delay in the first signal path branch.
The two directional microphone characteristics can be compared and, if necessary, processed by the addition of further parameters (for example, comparison to stored patterns of directional microphone characteristics) in order to select the directional microphone characteristic to be ultimately set.
The analysis and selection of the suitable directional microphone characteristic from among a number of available directional microphone characteristics can ensue with a selection unit, which can be programmable and may employ fuzzy logic or a neural network.
As a result, for example, acquired, different directional microphone characteristics can be compared to stored pattern characteristics, so that a user-friendly selection of the suitable directional microphone characteristic can ensue using decision rules.
The inventive hearing aid can include further signal path branches that are equipped with or without delay elements in order to be combined in respective summing elements and produce a number of directional microphone characteristics, from which the suitable directional microphone characteristic can then be selected. As warranted, a selected directional microphone characteristic can be adapted again by stored correction values, or mixtures of two or more directional microphone characteristics can also be realized.
In the inventive method, an A/D conversion is implemented in the signal path of at least one microphone by a sigma-delta converter, and the resulting 1-bit signal is delayed. As a result of the small word width of the output signal of the sigma-delta converter (1 bit) compared to the word width in the DSP unit (for example, 16 bits), considerably higher sampling rates can be used in the signal delay, so that a correspondingly more finely graduated delay can be realized. In the inventive method, the delay can ensue using a DSP control or with a shift register. If the signal in the signal path of at least one microphone passes through an interpolation filter, preferably operating as a low-pass filter, signal disturbances as may occur when switching between different delay rates can be attenuated or avoided.
The inventive method makes it possible to sample the signal to be delayed with a higher sampling rate, whereby sampling frequencies of far above 20 kHz can be achieved. The respective sampling frequency or the frequency range for matching to the specific requirements of the signal processing can be identified by trials. The inventive method is preferably implemented with elevated sampling frequencies of 400-800 kHz. Compared to a low-frequency sampling of, for example, 20 kHz wherein the samples can be shifted by 50 xcexcs, correspondingly more finely graduated shifts of 1.25 xcexcs-2.5 xcexcs are achieved by the elevated sampling rates of 400-800 kHz.
In further versions of the method, delayed and undelayed signal forwarding can occur in parallel branches in at least one microphone signal path, so that corresponding directional microphone characteristics that can be compared to one another are achieved by combining corresponding delayed and undelayed branch signals in summing elements.
Using, to the extent necessary stored decision rules or pattern characteristics, a suitable directional microphone characteristic can then be selected from a number of directional microphone characteristics or can be achieved by superimposition and adaptation of existing directional microphone characteristics.